This application is based on Japanese Patent Application HEI 11-346479 filed on Dec. 6, 1999, the entire contents of which are incorporated herein by reference.
a) Field of the Invention
The present invention relates to a solid-state image pickup device of CCD type and a method of producing the same.
b) Description of the Related Art
Since a mass-production technique of a charge-coupled device (CCD) has been developed, there have increasingly appeared apparatuses in which a solid-state image pickup device of CCD type (to be simply referred to as a solid-state image pickup device herebelow) is used as a line sensor (linear sensor) or an area image sensor.
A solid-state image pickup device for a line sensor includes a group of photoelectric converter elements formed in a line in a semiconductor substrate, a charge transfer path formed adjacent to the group of photoelectric converter elements on the substrate, and a plurality of readout gates to read signal charge from the photoelectric converter elements to feed the signal charge to the charge transfer path. When necessary, discharge gates are arranged to form an overflow drain of horizontal type.
The photoelectric converter element is generally composed of a photodiode.
The charge transfer path in the solid-state image pickup device for a line sensor is composed, for example, of a CCD of two-phase driving type. The charge transfer path includes a charge transfer channel formed in the semiconductor substrate and a plurality of transfer electrodes intersecting or crossing the charge transfer channel in a plan view.
The charge transfer channel includes, for example, two potential barrier regions and two potential well regions (storage regions) for one photoelectric converter element. The potential barrier and well regions are alternately formed in this order beginning at a position on an upstream side.
In this specification, movement of the charge through the charge transfer path is assumed to be a flow and a relative position of each constituent member or the like of the solid-state image pickup device is expressed as, for example, xe2x80x9cdownstream of member Xxe2x80x9d or xe2x80x9cupstream of member Yxe2x80x9d.
Each transfer electrode constituting the charge transfer path is arranged over the potential barrier or well region. Among four transfer electrodes arranged for one photoelectric converter element, two upstream transfer electrodes are commonly connected to each other and two downstream transfer electrodes are commonly connected to each other.
One readout gate is formed for each photoelectric converter element. Each readout gate includes a readout gate region formed in the semiconductor substrate to be contiguous to the associated photoelectric converter element and an associated charge transfer channel, and a readout gate electrode formed over the readout gate region. The readout gate electrodes are electrically connected to each other to constitute one electrode line.
Signal charge transferred through the charge transfer path is sent from the transfer path, for example, via an output gate or directly to a floating diffusion section or region. The signal charge is fed from the floating diffusion section to a predetermined source follower circuit. The signal charge is amplified by and is then outputted from the source follower circuit.
A reset transistor is formed using the floating diffusion section. The signal charge after the amplification (detection) is absorbed via a drain region of the reset transistor into a power source.
On the other hand, the solid-state image pickup device for an area image sensor includes a plurality of constituent units each of which includes (i) a group of photoelectric converter elements, (ii) a charge transfer channel, and (iii) one readout gate for each photoelectric converter element. A predetermined number of transfer electrodes are formed to intersect the charge transfer channel of each unit in a plan view.
One charge transfer channel and the transfer electrodes intersecting the charge transfer channel of each unit in a plan view constitute one charge transfer path. In an intersection region between each charge transfer channel and each transfer electrode in a plan view, a charge transfer stage is formed.
The transfer electrodes are composed only of transfer electrodes (to be referred to as readout transfer electrodes herebelow) each of which includes a section also serving as a readout gate electrode. Or, the transfer electrodes are composed of a plurality of transfer electrodes (to be referred to as transfer dedicated electrodes herebelow) each of which does not function as a readout gate electrode and a plurality of readout/transfer electrodes. The configuration of the transfer electrodes is decided based on a specification of arrangement of the photoelectric converter elements or the like.
The number of transfer electrodes for each photoelectric converter element is appropriately determined according to a specification of arrangement of the photoelectric converter elements, a method of driving the charge transfer paths, and the like. The charge transfer path is composed of a CCD of, for example, three-phase, four-phase, or eight-phase driving type.
The solid-state image pickup device for an area image sensor further includes an output transfer path. The output transfer path is connected to each of the charge transfer paths. The output transfer path is composed of a CCD of, for example, three-phase or four-phase driving type.
At a downstream end section of the output transfer path, a floating diffusion section is formed. The section is a constituent component of a reset transistor. When necessary, a discharge gate is disposed to form a horizontal overflow drain.
In the solid-state image pickup device for a line sensor, each transfer electrode, each readout gate electrode, each discharge gate electrode, and the gate electrode of the reset transistor are formed on a semiconductor substrate with an electrically insulating film between each of the electrodes and the semiconductor substrate. This also applies to the solid-state image pickup device for an area image sensor.
The semiconductor substrate of the solid-state image pickup device is a silicon substrate or a complex substrate in which a silicon layer is formed on one surface of a substrate made of quartz glass, or the like in general. The electrically insulating film to electrically isolate the semiconductor substrate from each transfer electrode and each gate electrode is, for example, a film made of silicon oxide, an ON film or,an ONO film.
In this specification, the ON film indicates a two-layer electronically insulating film being composed of an oxide layer formed on the semiconductor substrate and a nitride layer formed on the oxide layer. The ONO film indicates a three-layer electronically insulating film being composed of an oxide layer formed on the semiconductor substrate, a nitride layer formed on the oxide layer, and an oxide layer formed on the nitride layer.
Each of the electrodes is made of, for example, a first layer of polycrystalline silicon (1-polysilicon layer) or a second layer of polycrystalline silicon (2-polysilicon layer).
In the electrode constitution, when a film of silicon oxide (to be referred to as xe2x80x9cinsulating film Ixe2x80x9d in some cases herebelow) to electrically the electrodes from the semiconductor substrate is beforehand disposed on the semiconductor substrate, two phenomena or events may possibly occur as follows.
In a process to form an oxidized surface-layer of the 1-polysilicon layer to electrically isolate an electrode made of the 1-polysilicon layer from an electrode made of a 2-polysilicon layer, an insulating film I grows at a location in which the 2-polysilicon layer is to be formed. Resultantly, film thickness of the insulating film I below, the electrode made of the 2-polysilicon layer become greater than that of the insulating film I below the electrode made of the 1-polysilicon layer. This phenomenon will be referred to as phenomenon A1 herebelow.
In the process to form the oxidized surface-layer, the insulating film I grows below an end section of the 1-polysilicon layer and peels or warps an edge of the end section of the 1-polysilicon layer. This phenomenon will be referred to as phenomenon A2 herebelow. Phenomena A1 and A2 will be collectively referred to as xe2x80x9cphenomenon Axe2x80x9d herebelow.
When phenomenon A occurs in the production of a solid-state image pickup device for a line sensor, a threshold voltage for a desired operation varies between the transfer electrodes or between the readout gates. Resultantly, a line sensor having desired electric characteristics cannot be obtained. For example, transfer efficiency is lowered. The electric characteristics vary between the line sensors in some cases. This is also the case with a solid-state image pickup device for an area image sensor.
Therefore, when it is desired to form an electrode using a 1-polysilicon layer and an electrode using a 2-polysilicon layer on a semiconductor substrate, ON film or ONO film is often used as a material of the insulating film to insulate the electrodes from the semiconductor substrate.
The ON film used for the insulating film prevents phenomenon A in the forming of the oxidized surface-layer of the 1-polysilicon layer.
Also the ONO film used for the insulating film can prevent phenomenon A in the forming of the oxidized surface-layer of the 1-polysilicon layer.
To feed signal charge from a photoelectric converter element to the charge transfer path, a readout pulse having a relatively high voltage of, for example, about 15 V is applied to the readout gate electrode. A relatively high voltage of, a relatively high voltage ranging, for example, from 10 V to 20 V is also applied to the discharge gate electrode to discharge unnecessary charge and to the gate electrode of the reset transistor to discharge the charge after the amplification (detection).
When such a high voltage is applied to an electrode formed on the ON film, electrons are possibly trapped in the nitride film of the ON film or in a phase boundary between the nitride film and the oxide film of the ON film. This is also the case with a solid-state image pickup device in which an electrically insulating film made of an ONO film is used.
When electrons are trapped in the insulating film made of the ON or ONO film, the potential of each readout gate is changed in a solid-state image pickup device for a line sensor. For solid-state image pickup device for an area image sensor, the potential of each charge transfer stage constituted with a readout/transfer electrode or the potential of each readout gate is changed.
This results in variations in electric characteristics of the solid-state image pickup device. For example, a threshold voltage to drive the device changes with a lapse of time.
It is therefore an object of the present invention to provide a solid-state image pickup device in which variations in electric characteristics can be easily suppressed.
Another object of the present invention is to provide a method of producing a solid,-state image pickup device in which variations in electric characteristics can be easily suppressed.
According to one aspect of the present invention, there is provided a solid-state image pickup device, comprising: a semiconductor substrate; a photoelectric converter element group including a plurality of photoelectric converter elements formed in one column in one surface of said semiconductor substrate; a charge transfer channel formed in said semiconductor substrate, said charge transfer channel being adjacent to said photoelectric converter element group; a readout gate region formed for each said photoelectric converter element, each said readout gate region being contiguous to said associated photoelectric converter element and to said charge transfer channel; a plurality of transfer electrodes formed over said charge transfer channel with an ON film or an ONO film therebetween, each said transfer electrode intersecting said charge transfer channel in a plan view, said transfer electrodes being formed adjacent to each other and constituting one charge transfer path together with said charge transfer channel; and a readout gate electrode formed for each said readout gate region, each said readout gate electrode being formed over said associated readout gate region with an oxide insulating film therebetween, said readout gate electrodes being electrically connected to each other to form one electrode line.
According to another aspect of the present invention, there is provided a solid-state image pickup device, comprising: a semiconductor substrate; a plurality of photoelectric converter element groups formed in one surface of said semiconductor substrate, each said photoelectric converter element group including a plurality of photoelectric converter elements formed in one column in said semiconductor substrate; a charge transfer channel formed in said semiconductor substrate for each said photoelectric converter element group, said charge transfer channel being adjacent to said associated photoelectric converter element group; a readout gate region formed for each said photoelectric converter elements, each said readout gate region being contiguous to said associated photoelectric converter element and to one of said charge transfer channels; a plurality of transfer electrodes formed for each said charge transfer channel, each said transfer electrode being formed over each associated one of said charge transfer channels with an ON film or an ONO film therebetween, said transfer electrodes corresponding to one of said charge transfer channels being adjacent to each other and intersecting said one charge transfer channel in a plan view, said transfer electrodes corresponding to said one charge transfer channel constituting one charge transfer path together with said one charge transfer channel; and a readout gate electrode formed for each said readout gate region, each said readout gate electrode being formed over said associated readout gate region with an oxide insulating film therebetween, said readout gate electrodes corresponding to said photoelectric converter elements belonging to one of said photoelectric converter element groups being electrically connected to each other to form one electrode line.
According to still another aspect of the present invention, there is provided a method of producing a solid-state image pickup device, comprising: a preparation step of preparing a semiconductor substrate including in one surface thereof (a) a photoelectric converter element group including a plurality of photoelectric converter elements formed in one column, (b) a charge transfer channel formed adjacent to said photoelectric converter element group, and (c) a readout gate region formed for each said photoelectric converter element, each said readout gate region being contiguous to said associated photoelectric converter element and to said charge transfer channel; a lamination film forming step of forming an ON film or an ONO film on said one surface of said semiconductor substrate; a depression region forming step for locally removing at least a nitride layer from the ON film or a nitride layer and an oxide layer on the nitride layer from the ONO film and thereby forming a first depression region over the readout gate regions; an oxide insulating film forming step of forming an oxide insulating film on a bottom surface of the first depression region or forming an oxide insulating film by growing an oxide layer of a bottom of the first depression region; and an electrode forming step of forming, on the ON film or the ONO film, a plurality of transfer electrodes intersecting said charge transfer channel in a plan view and forming a readout gate electrode on the oxide insulating film for each said readout gate region.
According to still another aspect of the present invention, there is provided a method of producing a solid-state image pickup device, comprising: a preparation step of preparing a semiconductor substrate including in one surface thereof (a) a plurality of photoelectric converter element groups formed in the semiconductor substrate, each of the photoelectric converter element groups including a plurality of photoelectric converter elements formed in one column in the semiconductor substrate, (b) a charge transfer channel formed in the semiconductor substrate for each said photoelectric converter element group, the charge transfer channel being adjacent to the associated photoelectric converter element group, and (c) a readout gate region formed for each said photoelectric converter element, each of the readout gate regions being contiguous to the associated photoelectric converter element and to one of the charge transfer channels; a lamination film forming step of forming an ON film or an ONO film on said one surface of the substrate; a depression region forming step for locally removing at least a nitride layer from the ON film or a nitride layer and an oxide layer on the nitride layer from the ONO film and thereby forming a first depression region over each of the readout gate regions; an oxide insulating film forming step of forming an oxide insulating film on a bottom surface of the first depression region or forming an oxide insulating film by growing an oxide layer in a bottom of the first depression region; and an electrode forming step of forming a plurality of transfer electrodes over each of the charge transfer channels with the ON film or the ONO film therebetween, the transfer electrodes intersecting an associated charge transfer channel in a plan view and forming a readout gate electrode over each said readout gate region with the oxide insulating film therebetween.
In the above mentioned solid-state image pickup devices, each readout gate electrode is electrically isolated from the semiconductor substrate by the oxide insulating film. Consequently, there rarely occurs the event in which when a readout pulse is applied to the readout gate electrode, electrons are trapped by the insulating film (oxide insulating film) below the electrode.
Each transfer electrode is formed on the ON or ONO film. Therefore, at least occurrence of the phenomenon A2 can be suppressed even when the semiconductor substrate is a silicon substrate and each transfer electrode and each readout gate electrode are made of polycrystalline silicon layers. Phenomena A1 and A2, i.e., phenomenon A can be suppressed by using the 1-polysilicon layer for the readout gate electrode.
In consequence, it is possible in the solid-state image pickup device to easily suppress variations in electric characteristics thereof with a lapse of time. Between solid-state image pickup devices, variations in the electric characteristics thereof can be easily minimized.
According to still another aspect of the present invention, there is provided a solid-state image pickup device, comprising: a semiconductor substrate; a photoelectric converter element group including a plurality of photoelectric converter elements formed in one column in one surface of said semiconductor substrate; a charge transfer channel formed in said semiconductor substrate, said charge transfer channel being adjacent to said photoelectric converter element group; a readout gate region formed for each said photoelectric converter element, each said readout gate region being contiguous to said associated photoelectric converter element and to said charge transfer channel; a plurality of read/transfer electrodes intersecting said charge transfer channel in a plan view, each read/transfer electrode including a readout gate electrode section which covers said readout gate region in a plan view and which constitutes one readout gate together with said readout gate region and a transfer stage forming section which covers part of said charge transfer channel in a plan view and which constitutes one charge transfer stage together with said charge transfer channel, said one charge transfer stage being contiguous to said readout gate; a plurality of transfer dedicated electrodes intersecting said charge transfer channel in a plan view, said readout/transfer electrode and said transfer dedicated electrode being alternately formed, each said transfer dedicated electrode including a transfer stage forming section which covers part of said charge transfer channel in a plan view and which constitutes a charge transfer stage together with said charge transfer channel; an oxide insulating film intervening between said readout gate electrode section and said semiconductor substrate and between said transfer stage forming section in said readout/transfer electrode and said semiconductor substrate; and an ON film or an ONO film intervening between said transfer stage forming section in said transfer dedicated electrode and said semiconductor substrate.
In the above mentioned solid-state image pickup device, each readout gate electrode region is electrically isolated from the semiconductor substrate by the oxide insulating film. In the readout/transfer electrode, each transfer stage forming section is also electrically isolated from the semiconductor substrate by the oxide insulating film. Consequently, there rarely occurs the event in which when a readout pulse is applied to the readout/transfer gate electrode, electrons are trapped by the insulating film (oxide insulating film) below the electrode.
The transfer stage forming section in the transfer dedicated electrode is formed on the ON or ONO film. Therefore, even when the semiconductor substrate is a silicon substrate and each read/transfer electrode and each transfer dedicated electrode are made of polycrystalline silicon layers, at least occurrence of the phenomenon A2 can be suppressed. Phenomena A1 and A2, i.e., phenomenon A can be suppressed by using the 1 -polysilicon layer for the readout/transfer gate electrode.
In consequence, it is possible in the solid-state image pickup device to easily suppress variations in electric characteristics thereof with a lapse of time. Between solid-state image pickup devices, variations in the electric characteristics thereof can be easily minimized.